The present invention relates to a voice encoder using a voice activity detector for use in a voice communication system.
Portable radio terminals, such as digital cordless telephone apparatus, employ VOX (Voice Operate Switch Exchange) control which actuates a transmitter only during voice activity and holds it out of operation during a silent duration so as to reduce power consumption during transmission, and this control reduces the mean power consumption for transmission by about 15%. To perform such a VOX function, a voice activity detector for detecting the presence or absence of a voice signal needs to be provided at a stage preceding a transmitter output circuit.
The following will be described on the assumption that such a voice activity detector is applied to VOX control of a digital cordless telephone apparatus. The digital cordless telephone utilizes a 32 kb/s adaptive differential pulse code modulation (ADPCM) system as the voice coding system (CODEC), and the processing delay time in this apparatus is required to be equal to or shorter than 7 msec.
Since the processing by a conventional voice activity detector described below is executed for each 20 msec frame, a delay time of at least 20 msec is induced, making it impossible to meet a requirement that the delay time be 7 msec or less. Moreover, the conventional voice activity detector is formed independently of the voice encoder, and hence is defective in that the amount of data to be processed is inevitably large.